This invention relates to an electrolytic cell and to an electrode therefor and, in particular, to an electrolytic cell which is provided with liquor recirculating means.
Electrolytes, for example, aqueous solutions of alkali metal chlorides, particularly sodium chloride, are electrolyzed on a vast scale throughout the world in order to produce products such as chlorine and aqueous alkali metal hydroxide solution. The electrolysis may be effected in an electrolytic cell comprising a plurality of anodes and cathodes with each anode being separated from the adjacent cathode by a separator which divides the electrolytic cell into a plurality of anode and cathode compartments.
The electrolytic cell, may be of the diaphragm or membrane type. In the diaphragm type cell, separators positioned between adjacent anodes and cathodes are microporous and in use aqueous electrolyte passes through the diaphragms from the anode compartments to the cathode compartments of the cell. In the membrane type cell, the separators are essentially hydraulically impermeable, and in use, ionic species are transported across the membranes between the anode compartments and the cathode compartments of the cell.
For example, where aqueous alkali metal chloride solution is electrolyzed in an electrolytic cell of the diaphragm type the solution is charged to the anode compartments of the cell, chlorine which is produced during the electrolysis is removed from the anode compartments of the cell; the alkali metal chloride solution passes through the diaphragms, and hydrogen and alkali metal hydroxide produced by electrolysis are removed from the cathode compartments, the alkali metal hydroxide being removed in the form of an aqueous solution of alkali metal chloride and alkali metal hydroxide. Where an aqueous alkali metal chloride solution is electrolyzed in an electrolytic cell of the membrane type, the solution is charged to the anode compartments of the cell and chlorine produced during the electrolysis and depleted alkali metal chloride solution are removed from the anode compartments; alkali metal ions are transported across the membranes to the cathode compartments of the cell to which water or dilute alkali metal hydroxide solution may be charged; and hydrogen and alkali metal hydroxide solution produced by reaction of alkali metal ions with water are removed from the cathode compartments of the cell.
The electrolysis may be effected in an electrolytic cell of the filter press type which may comprise a large number of alternating anodes and cathodes, for example, fifty anodes alternating with fifty cathodes, although the cell may comprise even more anodes and cathodes, for example up to one hundred and fifty alternating anodes and cathodes.
The electrolytic cell may be provided with an inlet header through which electrolyte, for example aqueous alkali metal chloride solution, may be charged to the anode compartment of the cell, and with an outlet header through which products of electrolysis may be removed therefrom. Also, the electrolytic cell may be provided with an outlet header through which products of electrolysis may be removed from the cathode compartments of the cell, and optionally, e.g. in the case of a membrane type cell, with an inlet header through which liquor, for example water or other fluid, may be charged thereto.
Electrolytic cells may be fitted with means for recirculating the liquors to the anode and/or cathode compartments of the cell. For example, in an electrolytic cell of the membrane type in which aqueous alkali metal chloride solution is electrolyzed and in which the solution is charged to the anode compartments of the cell through an inlet header and chlorine and depleted aqueous alkali metal chloride solution are removed therefrom through an outlet header, the electrolytic cell may be equipped with means for removing depleted aqueous alkali metal chloride solution from the anode compartments and recirculating the depleted solution, or a part thereof, back to the anode compartments of the cell for re-use therein. Prior to effecting the recirculation, the gaseous chlorine may be separated from the depleted alkali metal chloride solution, and the depleted solution may be mixed with alkali metal chloride or with fresh more concentrated aqueous alkali metal chloride solution prior to recirculation of the solution to the anode compartments.
Recirculation of the aqueous alkali metal chloride solution enables the solution to be re-used, and it ensures that a high conversion of alkali metal chloride may be effected without the conversion of alkali metal chloride may be effected without the conversion in a single pass through the anode compartments being so high that unacceptable concentration gradients result in the solution within the anode compartments of the cell, and between the solutions in different anode compartments of the cell, with consequent loss in current efficiency. Furthermore, as the solution removed from the cell is at a high temperature, the fresh solution may be at relatively low temperature. Indeed, it may be unnecessary to heat the fresh solution.
The electrolytic cell may be equipped with similar means by which aqueous alkali metal chloride solution may be removed from the cathode compartments, and the solution, or a part thereof, recirculated back to the cathode compartments.
The electrolytic cell may be provided with a recirculating means in which the solutions are recirculated within the anode or cathode compartments of the cell, rather than being removed from the compartments and recirculated back to the compartments. Such internal recirculating means are particularly useful in assisting in the elimination of concentration gradients within the solutions in the anode or cathode compartments of the cell which in turn results in an improvement in the current efficiency at which the electrolysis is effected.
Removal of solution from the anode or cathode compartments and recirculation back to the compartments may be effected by means of suitable pipework positioned externally of the electrolytic cell. For example, the outlet header from the anode or cathode compartments of the cell may be connected to a branched outlet pipe and part of the depleted solution removed from the compartments may be passed through the branched pipe to an inlet pipe, which is in turn connected to the inlet header of the anode or cathode compartments of the cell, and through which fresh solution may also be charged to the compartments of the cell. Part of the solution removed from the anode or cathode compartments of the electrolytic cell may be removed from the cell through the branched pipe.
An electrolytic ell having pipework positioned externally of the cell and through which solutions may be recirculated is described in U.S. Pat. No. 3,856,651. The recirculation system relies for its effectiveness on the gas-lift effect, and in the patent, there is described a bipolar cell having a tank positioned on top of the cell to which chlorine-containing aqueous sodium chloride solution is passed from the anode compartments of the cell. Chlorine is separated from the solution in the tank, and the solution is removed from the tank and mixed with fresh, more concentrated sodium chloride solution and returned to the anode compartments of the cell via an externally positioned pipe.
Recirculation of the solution may also be effected within the anode or cathode compartments of an electrolytic cell. Such recirculation may be effected by means of downcomers positioned in the compartments of the cell, for example, by means of a downcomer positioned between a pair of electrode plates in an electrode compartment of a cell. Such recirculation also relies for its effectiveness of the gas-life effect.
An electrolytic cell in which there is internal recirculation is described in U.S. Pat. No. 4,557,816. In the patent, there is described a duct which facilitates downward flow of electrolyte and which is positioned in a space to the rear of an electrode; the duct comprising a horizontal portion having a lower opening near the inlet for fresh electrolyte and a vertical portion in communication with the horizontal portion and having an upper opening near the outlet for the depleted electrolyte.